The invention relates generally to magnetic sensors and more specifically to a thermally stable magnetic sensor in a bridge configuration having giant magnetoresistive sensors (GMR sensors) as resistive elements in a bridge design. Each adjacent resistor leg has an opposite response to an applied field providing a high and stable output signal.
GMR structures are multilayer devices. GMR effect is based on a spin-dependent scattering of electrons and arises from the magnetic state of the overall layered device. GMR structures were first developed in 1991 and were called xe2x80x9cspin valvesxe2x80x9d. A basic spin valve typically consists of two ferromagnetic layers separated by a thin spacer or non-magnetic layer. Because of shape anisotropy, the magnetization of both of the ferromagnetic layers typically lies parallel in plane. Usually the magnetization in one ferromagnetic layer is fixed or pinned along a predetermined direction. The magnetization in the other or xe2x80x9cfreexe2x80x9d or reference layer is allowed to rotate under the influence of an external magnetic field.
Such magnetic field sensors are widely used in applications such as linear and rotary motion sensors, proximity detectors, and speed and position sensors in automobiles. Common methods of detecting magnetic fields include Hall sensors and Anisotropic Magnetoresistance (AMR) filed sensors. Hall sensors are generally useful only for detection of relatively high magnetic fields (from 100 to 1000 Oe). AMR sensors generally have a small linear range, low saturation field, and poor sensitivity.
Currently available magnetic field sensors made from GMR devices have been used in a variety of applications. However, in these applications, the performance of the devices under varying temperatures has proved to be unstable, i.e., the magneto resistant characteristic of the sensors change substantially as the temperature changes. Often designs fail to provide a useful voltage output.
Therefore, there remains a need for a magnetic sensor that can operate with linear output over its operating range, substantial sensitivity in fields from 10 to 1000 Oe, has an acceptable signal amplitude and is stable, i.e., its magneto-resistance does not substantially change as the temperature varied from about room temperature (25xc2x0 C.) to elevated temperatures (about 200xc2x0 C.).
One embodiment of the invention provides a magnetic sensor that includes a first opposing pair and a second opposing pair of resistive elements configured in a Wheatstone bridge. In the Wheatstone bridge, the first opposing pair have magnetic polarities that are opposite to that of the second opposing pair, wherein each resistive element is a synthetic antiferromagnetic (SAF) GMR sensor having a reference layer and a pinned layer of different thicknesses. Each adjacent resistor leg has an opposite response to an applied magnetic field. This response is equal in amplitude to a single element. The opposing pinned/reference layer structure is attained by forming SAF structures of specific designs. The total thickness of the reference layer and the pinned layer is about 8 to 50 xc3x85. The first opposing pair has a reference layer that is greater than about 1 xc3x85 or great than about 5 xc3x85 thicker than the pinned layer, and the second opposing pair has a pinned layer that is greater than about 1 xc3x85 or greater than about 5 xc3x85 thicker than the reference layer.
A second embodiment of the invention provides a magnetic sensor that includes a first and a second opposing pair of resistive elements configured in a Wheatstone bridge, wherein the first opposing pair have magnetic polarities that are opposite to that of the second opposing pair, wherein each of the resistive elements is a SAF GMR sensor, and wherein the first opposing pair has a bilayer structure, and the second opposing pair has a trilayer structure having similar thickness characteristics.
A third embodiment of the invention provides a magnetic sensor that includes a first and a second opposing pair of resistive elements configured in a Wheatstone bridge, wherein the first opposing pair have magnetic polarities that are opposite to that of the second opposing pair, wherein the first opposing pair include SAF GMR sensor that include a reference layer and a pinned layer of different thicknesses, and the second opposing pair includes standard antiferromagnetic GMR sensors having similar thickness characteristics.
A last embodiment is an SAF free layer with two magnetic layers sandwiched with an Ru layer. Depending on the thickness ratio between the two soft magnetic layers, the GMR responses will have different polarity even when the pinned direction is the same.
In these embodiments, the thickness of each layer in the structure can be sized to compensate for changes in the spin valve stack. The elements can be made in this way to improve magnetic stability and voltage output over the range of useful fields.